Inkjet imaging devices eject liquid ink from printheads to form images on an image receiving member. The printheads include a plurality of inkjets that are arranged in some type of array. Each inkjet has a thermal or piezoelectric actuator that is coupled to a printhead controller. The printhead controller generates firing signals that correspond to digital data for images. The frequency and amplitude of the firing signals correspond to the selective activation of the printhead actuators. The printhead actuators respond to the firing signals by ejecting ink drops onto an image receiving member to form an ink image that corresponds to the digital image used to generate the firing signals.
Throughout the life cycle of these inkjet imaging devices, the image generating ability of the device requires evaluation and, if the images contain detectable errors, correction. Missing inkjets or weak inkjets are an error condition that affects ink image quality. A missing inkjet is an inkjet that does not eject an ink drop in response to a firing signal. A weak inkjet is an inkjet that responds intermittently to a firing signal or that responds by ejecting ink drops having a mass that is less than the ink drop mass corresponding to the characteristics of the firing signal for the inkjet. Systems and methods have been developed that compensate for missing or weak inkjets, but the missing or weak inkjets must be detected before these systems and methods can be activated.
Current detection methods include a test pattern being formed on an image receiving member and then digital data of the test pattern on the surface are generated. In an offset imaging device, the image receiving member is a rotating drum or belt. The digital data are produced by illuminating the drum or belt surface and generating an electrical signal that corresponds to the intensity of the light reflected from the surface. The signal is generated by an electro-optical sensor that is positioned to receive light reflected from a small portion of the drum or belt surface. By arranging a plurality of electro-optical sensors across the width of the drum or belt, the entire width can be used to generate reflected light received by the electro-optical sensors. The responses of the electro-optical sensors produce a digital image corresponding to the ink image on the drum or belt. The ink drops on the surface reflect light at an intensity that is different than the positions on the surface that do not have ink.
Evaluating a digital image produced by illuminating an image drum or belt can be difficult because the surface may generate noise in the digital image. Detecting the portion of the image data corresponding to ink on the drum or belt is made more difficult by the amount of ink in the test pattern. The amount of ink in test patterns is deliberately kept small since the test pattern is wiped from the drum or belt and the ink is collected by a drum maintenance unit. The drum maintenance unit includes a supply of release agent, an applicator, and a wiper. The wiper is selectively moved into and out of engagement with the image drum or belt to remove residual ink and other debris from the drum surface. The removed release agent, ink, and debris are directed to a sump within the drum maintenance unit. Because the capacity of the sump in the drum maintenance unit is relatively small, test patterns are printed with small amounts of ink. Testing has shown, however, that certain jetting failure mechanisms can only be seen repeatedly when a larger amount of ink is flowed through the failed inkjet. For example, some internal contamination particles can be suspended in the ink within the inkjet. Sometimes, a larger amount of ink must flow through the inkjet to move these suspended particles into the aperture to block the flow. Other mechanisms may be more complex, such as contaminates partially hanging out of a jet, physical aperture defects, and/or insufficient anti-wetting coatings. These mechanisms can exhibit the same behavior in that the inkjet can work correctly when a small amount of ink is ejected, but can fail when used at a higher duty cycle with a larger amount of jetted ink mass. Consequently, the failed inkjet ejection process and/or the image processing required for detection of the actual number of failed inkjets can be significant and still be susceptible to error. Improving the ability of inkjet imaging systems to detect missing and weak inkjets in an inkjet imaging system remains important to such systems.